NARROW BAND EMITTING SiAlON PHOSPHOR

ABSTRACT

This specification discloses a method of enhancing the stability and performance of Eu2+ doped narrow band red emitting phosphors. The resulting phosphor compositions are characterized by crystallizing in ordered structure variants of the UCr4C4 crystal structure type and having a composition of AE1−xLi3−2yAl1+2y−zSizO4−4y−zN4y+z:EUx(AE=Ca, Sr, Ba; 0&lt;x&lt;0.04, 0≤y&lt;1, 0&lt;z&lt;0.05, y+z≤1). It is believed that the formal substitution (Al,O)+ by (Si,N)+ reduces the concentration of unwanted Eu3+ and thus enhances properties of the phosphor such as stability and conversion efficiency.

This application claims benefit of priority to U.S. Provisional PatentApplication 62/944,025 filed Dec. 5, 2019 and to European PatentApplication 20151188.8 filed Jan. 10, 2020, each of which isincorporated herein by reference in its entirety,

FIELD OF THE INVENTION

The invention relates generally to phosphors and phosphor-convertedlight emitting diodes, and more particularly to narrow band emitting SiAlON phosphors, to methods for making them, and to phosphor convertedlight emitting diodes comprising them.

BACKGROUND

Semiconductor light emitting diodes and laser diodes (collectivelyreferred to herein as “LEDs”) are among the most efficient light sourcescurrently available. The emission spectrum of an LED typically exhibitsa single narrow peak at a wavelength determined by the structure of thedevice and by the composition of the semiconductor materials from whichit is constructed. By suitable choice of device structure and materialsystem, LEDs may be designed to operate at ultraviolet, visible, orinfrared wavelengths.

LEDs may be combined with one or more wavelength converting materials(generally referred to herein as “phosphors”) that absorb light emittedby the LED and in response emit light of a longer wavelength. For suchphosphor-converted LEDs (“pcLEDs”), the fraction of the light emitted bythe LED that is absorbed by the phosphors depends on the amount ofphosphor material in the optical path of the light emitted by the LED,for example on the concentration of phosphor material in a phosphorlayer disposed on or around the LED and the thickness of the layer.

Phosphor-converted LEDs may be designed so that all of the light emittedby the LED is absorbed by one or more phosphors, in which case theemission from the pcLED is entirely from the phosphors. In such casesthe phosphor may be selected, for example, to emit light in a narrowspectral region that is not efficiently generated directly by an LED.

Alternatively, pcLEDs may be designed so that only a portion of thelight emitted by the LED is absorbed by the phosphors, in which case theemission from the pcLED is a mixture of light emitted by the LED andlight emitted by the phosphors. By suitable choice of LED, phosphors,and phosphor composition, such a pcLED may be designed to emit, forexample, white light having a desired color temperature and desiredcolor-rendering properties.

Phosphor converted LEDs that comprise narrow band red emitting phosphorsof composition A_(a−z)—B_(b)—C_(c)—X_(x):Eu_(z) with A=(Sr,Ba,Ca,La,Lu);B=(Li,Mg); C=(Si,Al,B,Ga,P,Ge); X=(N,O,S,F,Cl); and 0.5≤c/x≤0.75 showingfor example an eightfold coordination of the activator ion by itsligands and activator contact lengths in the 210-320 pm range aredisclosed in WO 2010/131133 A1. Examples of such Eu doped phosphormaterials are for example homeotypic SrLiAl₃N₄:Eu (SLA) orSrLi₂Al₂O₂N₂:Eu (SLAO) disclosed in U.S. Pat. No. 9,546,319 B2 and WO2018/087304 A1, respectively.

A known issue of such Eu doped phosphor materials is the tendency toincorporate the Eu activator not only in the preferred divalent statebut also in the non-wanted trivalent state. An article entitled“Pressure-Controlled Synthesis of High-Performance SrLiAl₃N₄:Eu²⁺Narrow-Band Red Phosphors” by Fang et al. (J. Mater. Chem. C, 2018, DOI:10.1039/C8TC03025A) teaches a process of elevating the gas pressureduring the synthesis, leading to a slightly decreased unit ceil volume,an increased quantum efficiency and an increased Eu²⁺/Eu³⁺ ratio,eventually enhancing the luminescence intensity of the SLA phosphormaterial.

The present inventors found however that phosphor materials like SLAOcannot be enhanced in their properties as is SLA by applying highprocess gas pressures. While in SLA the average alkaline earth cationsize can only be decreased by replacing part of Sr (increased chemicalpressure), the opposite has been observed by the present inventors forSLAO in which Ca is not soluble but the larger Ba atom (decreasedchemical pressure) is. Since the lattice compressibility should show thesame tendency as the unit cell volume change by cation substitutionanother Eu²⁺ stabilization mechanism is needed to provide SLAO typephosphor materials with improved properties.

SUMMARY

This specification discloses a method of enhancing the stability andperformance of Eu²⁺ doped narrow band red emitting phosphors. Theresulting phosphor compositions are characterized by crystallizing inordered structure variants of the UCr₄C₄ crystal structure type andhaving a composition ofAE_(1−x)Li_(3−2y)Al_(1+2y−z)Si_(z)O_(4−4y−z)N_(4y+z):EU_(x); (AE=Ca, Sr,Ba; 0<x<0.04, 0≤y<1, 0<z<0.05, y+z≤1). It is believed that the formalsubstitution (Al,O)⁺ by (Si,N)⁺ reduces the concentration of unwantedEu³⁺ and thus enhances properties of the phosphor such as stability andconversion efficiency.

These compositions may be viewed as a stabilized version of SLAOemitting in, for example, the 612-620 nm range with, for example,FWHM≤55 nm. They are believed to solve the issue of a low operationlifetime of pcLEDs comprising a narrow band red emitting SLAO typephosphor by changing the defect chemistry of the SLAO material byaddition of silicon to form novel SiAlON compositions that crystallizein the SLAO structure type.

Preferably, the Si concentration in the host lattice is in about thesame range as the Eu doping concentration.

One subgenus, with y=½ has compositionAE_(1−x)Li₂Al_(2−z)Si₂O_(2−z)N_(2+z):Eu_(x).

The novel phosphor compositions disclosed in this specification may beemployed, for example, in white light emitting pcLEDs, for example withcolor rendering index (CRT) of 90 or greater, and in red emittingpcLEDs.

Other embodiments, features and advantages of the present inventionwill, become more apparent to those skilled in the art when taken withreference to the following more detailed description of the invention inconjunction with the accompanying drawings that are first brieflydescribed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-sectional view of an example pcLED.

FIGS. 2A and 2B show, respectively, cross-sectional and top schematicviews of an array of pcLEDs.

FIG. 3A shows a schematic top view of an electronics board on which anarray of pcLEDs may be mounted, and FIG. 3B similarly shows an array ofpcLEDs mounted on the electronic board of FIG. 3A.

FIG. 4A shows a schematic cross sectional view of an array of pcLEDsarranged with respect to waveguides and a projection lens. FIG. 4B showsan arrangement similar to that of FIG. 4A, without the waveguides.

FIG. 5 shows a powder x-ray diffraction spectrum for the phosphorproduct of comparative Example A, Sr_(0.995)Li₂Al₂O₂N₂:Eu_(0.005),described below.

FIG. 6 shows an emission spectrum for the phosphor product ofcomparative Example A.

FIG. 7 shows the thermal behavior of the emission intensity of thephosphor product of comparative Example A.

FIG. 8 shows a powder x-ray diffraction pattern for the synthesisproduct of Example B, reagent Eu₂Si₂N₈, described below.

FIG. 9 shows a powder x-ray diffraction pattern for Example C,Sr_(0.995)Li₂Al₂Si_(0.0125)O_(1.9875)N_(2.0125):Eu_(0.005), describedbelow.

FIG. 10 shows an emission spectrum for the phosphor product of ExampleC.

FIG. 11 shows the thermal behavior of the emission intensity of thephosphor product of Example C

DETAILED DESCRIPTION

The following detailed description should be read with reference to thedrawings, in which identical reference numbers refer to like elementsthroughout the different figures. The drawings, which are notnecessarily to scale, depict selective embodiments and are not intendedto limit the scope of the invention. The detailed descriptionillustrates by way of example, not by way of limitation, the principlesof the invention.

FIG. 1 shows an example of an individual pcLED 100 comprising a lightemitting semiconductor diode structure 102 disposed on a substrate 104,together considered herein an “LED”, and a phosphor layer 106 disposedon the LED. Light emitting semiconductor diode structure 102 typicallycomprises an active region disposed between n-type and p-type layers.Application of a suitable forward bias across the diode structureresults in emission of light from the active region. The wavelength ofthe emitted light is determined by the composition and structure of theactive region.

The LED may be, for example, a III-Nitride LED that emits blue, violet,or ultraviolet light. LEDs formed from any other suitable materialsystem and that emit any other suitable wavelength of light may also beused. Other suitable material systems may include, for example,III-Phosphide materials, III-Arsenide materials, and II-VI materials.

Any suitable phosphor materials may be used, depending on the desiredoptical output from the pcLED.

FIGS. 2A-2B show, respectively, cross-sectional and top views of anarray 200 of pcLEDs 100 including phosphor pixels 106 disposed on asubstrate 202. Such an array may include any suitable number of pcLEDsarranged in any suitable manner. In the illustrated example the array isdepicted as formed monolithic-ally on a shared substrate, butalternatively an array of pcLEDs may be formed from separate individualpcLEDs. Substrate 202 may optionally comprise CMOS circuitry for drivingthe LED, and may be formed from any suitable materials.

As shown in FIGS. 3A-3B, a pcLED array 200 may be mounted on anelectronics board 300 comprising a power and control module 302, asensor module 304, and an LED attach region 306. Power and controlmodule 302 may receive power and control signals from external sourcesand signals from sensor module 304, based on which power and controlmodule 302 controls operation of the LEDs. Sensor module 304 may receivesignals from any suitable sensors, for example from temperature or lightsensors. Alternatively, pcLED array 200 may be mounted on a separateboard (not shown) from the power and control module and the sensormodule.

Individual pcLEDs may optionally incorporate or be arranged incombination with a lens or other optical element located adjacent to ordisposed on the phosphor layer. Such an optical element, not shown inthe figures, may be referred to as a “primary optical element”. Inaddition, as shown in FIGS. 4A-4B a pcLED array 200 (for example,mounted on an electronics board 300) may be arranged in combination withsecondary optical elements such as waveguides, lenses, or both for usein an intended application. In FIG. 4A, light emitted by pcLEDs 100 iscollected by waveguides 402 and directed to projection lens 404.Projection lens 404 may be a Fresnel lens, for example. This arrangementmay be suitable for use, for example, in automobile headlights. In FIG.4B, light emitted by pcLEDs 100 is collected directly by projection lens404 without use of intervening waveguides. This arrangement mayparticularly be suitable when pcLEDs can be spaced sufficiently close toeach other, and may also be used in automobile headlights as well as incamera flash applications. A mieroLED display application may usesimilar optical arrangements to those depicted in FIGS. 4A-4B, forexample. Generally, any suitable arrangement of optical elements may beused in combination with the pcLEDs described herein, depending on thedesired application.

As summarized above, this specification discloses a novel phosphorcomposition with superior luminescence properties compared to knowphosphor compositions with isotypic crystal structures.

In particular, the inventors found that an improved SLAO type phosphormaterial can be obtained by co-doping the host lattice with Si to formSiAlON compounds. It is believed by the inventors that the formalsubstitution (Al,O)⁺ by (Si,N)⁺ reduces the concentration of unwantedEu³⁺ by increasing the concentration of the highly charged Si⁴⁺ in thehost lattice and thus enhances the stability and conversion efficiencyof the phosphor material by suppressing the formation of Eu³⁺ byoxidation of the Eu²⁺ dopant. This is advantageous because a lowertendency to form unwanted Eu³⁺ during operation of a phosphor convertedLED comprising an SLAO type phosphor is desired to increase thereliability of such a device.

Specifically, the phosphor materials have compositionAE_(1−x)Li_(3−2y)Al_(1+2y−z)Si_(z)O_(4−4y−z)N_(4y+z):Eu_(x) (AE=Ca, Sr,Ba; 0<x<0.04, 0.4<y<0.6, 0<z<0.05), where part of the aluminum of thehost lattice is being replaced by silicon to form Si AlON type offormulations. To maintain charge neutrality of the host lattice also apart of the oxygen atoms are being replaced by nitrogen atoms. In otherwords, (Al,O)⁺ pairs are being replaced charge neutral by (Si,N)⁺ pairsin the phosphor host lattice.

The Si concentration should be in the range of the Eu activatorconcentration. If the activator concentration x is for example 0.005,the Si concentration should preferably be in the range 0.001 to 0.02,more preferably in the range 0.0025 to 0.015. More generally, referringto the formula above characterizing the phosphor composition, preferably⅕≤z/x≤4; more preferably ½≤z/x≤3.

A preferred option to incorporate Si into the phosphor to form a Si AlONcomposition is via a nitride material such as for example siliconnitride. Even more preferred is the incorporation via Eu₂Si₅N₈ whichalso acts as Eu precursor with Eu in the divalent state and a Eu/Siratio in the preferred range. The inventors found that Eu₂Si₅N₈ can beeasily prepared from commercially available europium oxide, carbon andsilicon nitride powders. Eu₂Si₅N₈ can be used as the only source of Eudopant or it can be mixed with other sources like, for example, Eu₂O₃,EuF₃ or EuN.

In the following, examples for carrying out the invention are given.

Example A—Comparative Example, Synthesis ofSr_(0.995)Li₂Al₂O₂N₂:Eu_(0.005)

30.312 g Strontium hydride (Materion, 99.5%), 17.202 g Lithium aluminumnitride prepared from Lithium nitride (Materion, 99.5%) and aluminumnitride (Tokuyama, grade F), 23.1746 g aluminum oxide (Baikowski,SP-DBM), 0.2988 g europium oxide (Neo, 4N), and 0.3733 g lithiumfluoride (Aldrich, 99.99%) are mixed in a ball mill and fired at 730° C.setting temperature under nitrogen in a graphite furnace for 24 hrs.After ball milling in ethanol, the phosphor powder is dried and screenedby sieving. FIG. 5 shows the x-ray diffraction (XRD) powder pattern ofthe powder product, indicating that it crystallizes in the tetragonalcrystal structure of SLAO with lattice parameters a₀=7.950 Å andc₀=3.183 Å.

The powder shows a peak emission at 618 nm with an emission half widthof 53 nm if excited with 440 nm blue light (FIG. 6). The emissionstability is assessed by heating a powder sample in air under 450 nmillumination and monitoring the emission intensity. The sample is firstheated to 300° C. in 25K steps with 20 min dwell times and then cooleddown. The drop in emission intensity with temperature is due to thermalquenching of the luminescence. After cooling down the emission signal isnot fully recovered. FIG. 7 shows the relative emission intensity of thepower sample as a function of the heating and cooling temperature. A 5%loss due to irreversible degradation is observed.

Examples—Synthesis of Eu₂Si₅N₈

37.3 g silicon nitride (USE, >98.5%), 57.3 g europium oxide (NEO,99.99%) and 6.45 g graphite (Alfa Aesar, microcrystal grade) are mixedby ball milling in cyclohexene, dried and transferred into a tubefurnace. After firing at 1550° C. under a forming gas atmosphere (5% H₂,95% N₂) for 8 h, the resulting Eu₂Si₅N₈ powder is ball milled inisopropanol and finally dried. FIG. 8 shows the XRD powder pattern ofthe Eu₂Si₅N₈ powder, indicating that it crystalizes in an orthorhombiclattice of the Ba₂Si₅N₈ structure type with cell constants a₀=5.7125 Å,b₀=6.793 Å, c₀==9.347 Å.

Example C—Synthesis ofSr_(0.995)Li₂Al₂Si_(0.0125)O_(1.9875)N_(2.0125):Eu_(0.005)

30.324 g Strontium hydride (Materion, 99.5%), 17.185 g Lithium aluminumnitride prepared from Lithium nitride (Materion, 99.5%) and aluminumnitride (Tokuyama, grade F), 23.111 g aluminum oxide (Baikowski,SP-DBM), 0.473 g europium nitridosilicate (from Example B), and 0.307 glithium fluoride (Aldrich, 99.99%) are mixed in a ball mill and fired at730° C. setting temperature under nitrogen in a graphite furnace for 24hrs. After ball milling in ethanol, the phosphor powder is dried andscreened by sieving. FIG. 9 shows the XRD powder pattern of the powderproduct, indicating that it crystallizes in the tetragonal crystalstructure of SLAO with lattice parameters a₀==7.948 Å and c₀==3.185 Å.

FIG. 10 shows the emission spectrum of the powder for 440 nm excitationwavelength. The emission shows a maximum at 616 nm and an emission halfwidth of 54 nm. FIG. 11 shows the thermal behavior of the emissionintensity of the powder for this example as measured with the samemethod as used for Example C. While the heating up part, of themeasurement, is nearly identical with the one obtained for Example A,the irreversible degradation of the emission intensity is significantlylowered from 5% to 2%.

This disclosure is illustrative and not limiting. Further modificationswill be apparent to one skilled in the art in light of this disclosureand are intended to fall within the scope of the appended claims.

1. A luminescent composition of matter characterized by the formula:AE_(1−x)Li_(3−2y)Al_(1+2y−z)Si_(z)O_(4−4y−z)N_(4y+z):EU_(x); AE=Ca, Sr,Ba; 0<x<0.04; 0.4<y<0.6; and 0<z<0.05).
 2. The luminescent compositionof matter of claim 1, wherein y is equal to or about equal to 0.5. 3.The luminescent composition of matter of claim 1 characterized by theformula AE_(1−x)Li₂Al_(2−z)Si_(z)O_(2+z)N_(2+z):EU_(x).
 4. Theluminescent, composition of matter of claim 1, crystalized in a UCr₄C₄type crystal structure.
 5. The luminescent composition of matter ofclaim 1, emitting light with an emission peak having a peak wavelengthin the range 612 nm to 620 nm and a full width at half maximum ≤55 nm.6. The luminescent composition of matter of claim 1, wherein ⅕≤z/x≤4. 7.The luminescent composition of matter of claim 6, wherein ½≤z/x≤3. 8.The luminescent composition of matter of claim 1 wherein y is equal toor about equal to 0.5, crystalized in a UCr₄C₄ type crystal structure.9. The luminescent composition of matter of claim 8, emitting light withan emission peak having a peak wavelength in the range 612 nm to 620 nmand a full width at half maximum ≤55 nm.
 10. The luminescent compositionof matter of claim A9, wiierein ⅕≤z/x≤4.
 11. A method for making theluminescent composition of matter of claim 1, comprising reactingEu₂Si₅N₈ with additional reagents.
 12. A light emitting devicecomprising: a semiconductor light emitting device; and the phosphorcomposition of claim 1 positioned to absorb light emitted by thesemiconductor light emitting device and in response emit light of alonger wavelength.
 13. The light emitting device of claim 12, whereinthe phosphor composition of claim A1 is positioned on or adjacent thesemiconductor light emitting device.
 14. The light emitting device ofclaim 12, wherein the phosphor composition of claim A1 is positionedremote from the semiconductor light emitting device.
 15. The lightemitting device of claim 12, wherein a combined light output from thesemiconductor light emitting device and the phosphor composition ofclaim A1 is perceived as white by a human with normal color vision. 16.The light emitting device of claim 15, wherein the combined light outputfrom the semiconductor light emitting device and the phosphorcomposition of claim A1 is characterized by a CRI of 90 or greater.